The present invention relates to a pressure cooker, particularly one which may have its cover opened only when the pressure in it has fallen to a safe level.
A pressure cooker can form an air-tighted space in a cooker body by an airtight connection between cooker body and cover so that the temperature and the pressure in the cooker can rise rapidly for shortening the cooking time. The cover may be removed only after lowering the pressure through a relief valve.
Seals between a cooker body and a cover as well as relief valve mechanism are quite reliable after experiencing improvements for many years. However, some safety problems still exist, such as:
(1) When food of a high expansion coefficient is cooked in a pressure cooker, the food expands quickly and particles may clog the relief valve before the relief valve functions. Then, high pressure in the cooker may cause explosion, or cause injury to persons when its cover is opened. Putting an excessive quantity of food in a pressure cooker may also cause hazards. Imperfect safety design of the conventional pressure cooker is a main cause of accidents arising in the use of a pressure cooker.
(2) One known relief valve of the conventional design begins when the pressure relief only pressure in the cooker reaches a certain level. It does not give a restriction to the timing of cover opening. Therefore, the cover of the pressure cooker may be opened before pressure is lowered to a safe level, and thus, an accident may occur.
(3) Air-tightness and high pressure resistance are required in the connection between a cooker body and its cover. Therefore, the conventional pressure cooker is usually very heavy and not convenient in use.
U.S. Pat. No. 2,237,952 granted to Smith on Apr. 8, 1941 is for an arrangement of leakproof cover and ring seal for forming a closed space in a cooker in order to improve thermal efficiency and rapidly raise the temperature and pressure in the cooker, but it has the following defects: (1) there is no way to prevent opening of its cover under high pressure, and the cover must be opened very carefully to avoid an accident; (2) its structure is complicated and it is heavy; and (3) its relief valve is not a good safety device.
U.S. Pat. No. 3,029,724 granted to Lee on Apr. 17, 1962 discloses a pressure cooker which uses a clamping means to fix or release a lid means and a containers so that an air-tight space can be formed in the container. Its design includes a pressure gauge and a safety valve on the lip means. However, it has the following defects: (1) the safety valve could be out of order due to clogging of a highly expandable food in the cooker; and (2) there is no way to prevent opening of the lip means when there is a high pressure in the cooker.
U.S. Pat. No. 4,294,377 granted to the inventor on Oct. 13, 1983 discloses a constant pressure cooker and fastener to eliminate the possibility of relief valve clogging. It uses a elastic coupling steel ring to fit a cover and a cooker body in order to form an air-tight space in the cooker. Thus it has many merits, it does not any way prevent opening of the cooker when there is a high pressure in the cooker. Furthermore, due to limit of the elasticity of the coupling steel ring, pressure in the cooker will be relieved even at a level which is quite high.
Therefore, there is a need for safety measures to preclude opening of pressure cooker at high pressure and a need for a relief valve which can relieve pressure to a safe level. An alternate way of pressure relief is desirable too.